The invention is directed to an optical sensor and a method as well as their use in a machine for suppressing faults due to diffused light in optical sensors.
Optical sensors for detecting objects can be embodied, for example, as reflection light sensors or light bars. They comprise a light source for sending visible or invisible light and a detector for receiving light, which is emitted by the light source. light emitting diodes, laser diodes, or IR-diodes can be used as light sources, for example. Depending on the embodiment of the sensor, the light source may be continuous or, in order to minimize external light influences, operated in a pulsed or clocked fashion. It is also known to polarize the light and/or to focus it via blind or lenses to a light beam.
For example, phototransistors or photodiodes can be used as detectors. Depending on the purpose and operation of the sensors, the light source and the detectors may be arranged in a common housing or spatially separated from one another in separate housings.
In conventional reflection light sensors, the light source emits a light beam, which is reflected by an object and/or a measuring object in a mirror-like or diffuse manner. Here, the light is reflected exactly in one direction or is reflected into the room, with a homogenous or irregular distribution of the light intensity. A portion of the reflected light can be recognized by the detector and evaluated. In other words, the light spot created by the light beam on the object is imaged on the light-sensitive surface of the detector by an imaging optic arranged in front of the detector.
Simple reflection light sensors only evaluate the intensity of the detected light: the shorter the distance between the light source and the measuring object the higher the light intensity recognized by the detector. By determining a switching threshold a switching distance can be predetermined for a certain type of object measuring. The switching distance adjusted in this manner can be influenced in an undesired manner by various parameters, such as e.g., color and reflection features of the object and the background or contamination of the sensor.
Most conventional reflection light sensors with background masking as well as distance sensors operate according to the triangulation principle. Here, the light portion reflected by the object in the direction of the detector is imaged on the detector and evaluated depending on the position and/or location of the recognized light on the detector, which is changing by the distance between the sensor and the object. The detector is embodied such that it can distinguish at least two different impingement positions of the light reflected at a measuring object. For example, two or more photodiodes or phototransistors can be used, discretely positioned or integrated on a common substrate.
The diameter of the beam of the light emitted by the light source, either coherent or incoherent, can be limited by optical elements, such as blinds or lenses. Based on the reflection and/or diffusion effects and/or reflexes at or in the transmitting elements the edge of the light beam cannot be limited precisely and the light intensity outside the core beam is not negligibly small. Primarily in highly sensitive detectors this can lead to the situation that even the light of the edge zones surrounding the core beam is sufficient to activate an output level of the sensor. In the following, the light in the edge zone of the light beam is called corona or diffused light or diffraction light, independent if the light was created by diffraction and/or diffusion and/or reflections (secondary spots) of the light emitted by the light source. If the detector is provided, e.g., in the form of a CCD-array in the form of a matrix with high spatial resolution, the differences in luminescence on the individual pixels can be evaluated and the exact position of the core beam can be determined. In a linear CCD-array, this can be performed within the array; however, not laterally to the linear array. If the detector includes only one or only few photo elements, the position of the light spot imaged on the detector and/or, in reflecting objects, of the core beam reflected at the object cannot be determined or distinguished from the corona in this manner. Problems can particularly arise when the light beam is not entirely reflected by an object, e.g., when the light at the edge zone of the beam already impinges the object at an object edge and is at least partially reflected towards the detector, but the light of the core beam is not.